A Hybrid G-Quadruplex Structure Formed by a Sequence from the MAX Gene

Here, we report the NMR solution structure of the G-quadruplex (G4) folded by the 24-nt guanine-rich DNA sequence d-[AG₃AG₄A₂G₄TGA₂G₄A]. The sequence is originated from the first intron of the MAX gene, which encodes the transcription factor MAX, an obligate partner of the notorious oncoprotein MYC. The unimolecular G4 adopts a [3 + 1] hybrid topology containing all the three main loop types (propeller, diagonal, and lateral). Also, the three G-tetrads are stacked with the same polarity of the Hoogsteen hydrogen bond and <i>anti</i>-<i>anti</i>-<i>anti</i>-<i>syn</i> glycosidic configuration pattern. Both 5' and 3'-flanking regions contain one adenine residue. As far as we know, it is the first example of such a unique topology wherein all the nucleotides are not modified in either sugars or bases. In addition to the well-defined G-tetrad core, the diagonal loop contains a G·(A-G) triad, and a T·G base pair is formed within the lateral loop. Furthermore, the formation of the G4 is found to be stable under physiological conditions even without annealing and acts as a barrier to DNA polymerase. These findings provided deep insights into the G4 folding topology and a potential for targeting MAX G4.